1. Field of the Invention
The present invention relates, generally, to a system and method of calibrating a plasma device. More particularly, the present invention provides a non-linear test load for calibrating a multiple frequency plasma device in the absence of a plasma reaction to facilitate testing of the system under operating conditions.
2. Discussion of the Background
Plasma devices or “reactors” are widely utilized for deposition processing, as well as dry etching processing such as “ashing” and “sputtering” utilized in the manufacture of semiconductor devices. In use, the plasma device introduces “plasma” or ionized gas to serve as a conduction medium which is interposed between opposing electrodes (i.e., anode and cathode). A high voltage discharge is placed across the electrodes such that the conduction of electrons across the plasma gap between the anode and cathode causes a self sustained scattering of collisions with plasma molecules (collisional ionization) or “plasma reaction”, creating a cascading of charged particles at the surface of an opposite electrode. By positioning a semiconductor substrate at the opposing electrode, chemical reaction of the substrate with the bombarding particles (depending on chemistry of plasma and substrate) enables a selective deposition and/or etching process. In etching processes, this enables certain materials to be removed at a faster rate relative to others in accordance with known chemical reactions. The removed materials are often pumped away in the form of gas phase molecules.
While the discharge voltage can be driven by either an AC or DC source, AC is preferred over DC as AC signals are not as limited by capacitive charging transients and are especially efficient at higher frequencies such as in the radio frequency (RF) spectrum. In determining the necessary discharge and associated multi-frequency operational parameters for sustaining the plasma reaction, the spatial distribution of charges within a plasma must be considered across multiple frequencies.
Importantly, the two main spatial areas are (1) the “sheath” areas and (2) the gap areas. The sheath areas occur in the vicinity of each electrode (or other surface) contacting the plasma. These “sheath” areas are also sometimes referred to as “dark spaces.” The sheath areas are created by a regional accumulation of electrons and/or ions. (Most of the voltage applied to the plasma is dropped across the sheath regions.) The gap area is generally any region bounded by sheath areas that has a lower concentration of electrons and/or ions.
Probe circuitry may be utilized in conjunction with the plasma device to assess operational performance. Typically, a dummy load is placed in the device, and data is provided for a single frequency. Typically the single frequency is the fundamental frequency of the process. Yet, this data is of limited use as it is only directed to a single frequency, requires the presence of plasma, and is cost prohibitive to repeat for varying frequency values.